The Amazon dolphin, Inia geoffrensis, parted company with its oceanic ancestors about 15 million years ago, during the Miocene epoch. Sea levels were higher then, says biologist Healy Hamilton of the California Academy of Sciences in San Francisco, and large parts of South America, including the Amazon Basin, may have been flooded by shallow, more or less brackish water. When this inland sea retreated, Hamilton hypothesizes, the Amazon dolphins remained in the river basin, evolving into striking creatures that bear little resemblance to our beloved Flipper. These dolphins have fat, bulbous foreheads and skinny, elongated beaks suited to snatching fish from a tangle of branches or to rooting around in river mud for crustaceans. Unlike marine dolphins, they have unfused neck vertebrae that allow them to bend at up to a 90-degree angle—ideal for slithering through trees. They also have broad flippers, a reduced dorsal fin (a larger one would just get in the way in tight spots), and small eyes—echolocation helps them pinpoint prey in muddy water.

At up to 450 pounds and eight feet in length, the Amazon dolphin, or boto, is the largest of the four known species of river dolphin. The others live in the Ganges in India and the Indus in Pakistan, in the Yangtze in China, and in the Río de la Plata between Argentina and Uruguay. All river dolphins are superficially similar, says Hamilton, yet the four species don't belong to the same family. DNA studies by Hamilton and others have shown that river dolphins evolved from archaic marine cetaceans (the order that also includes whales) on at least three separate occasions—first in India, later in China and in South America—before modern marine dolphins themselves had emerged as a distinct group. In an example of what's known as convergent evolution, geographically isolated and genetically distinct species developed similar characteristics because they were adjusting to similar environments.

Every spring the Amazon dolphins get to leave the confines of their river channel for a taste of their former habitat. At the Mamirauá Reserve in western Brazil, where Tony Martin of the University of Kent in England has studied the dolphins for the past 16 years, two Amazon tributaries flood thousands of square miles of forest for half the year, converting it into a vast tree-canopied sea. Martin and his Brazilian colleague Vera da Silva have found that female dolphins in particular stray far into the forest—perhaps to take refuge from aggressive, bright pink males. The females are mostly gray; the males' pink color, Martin and da Silva believe, is scar tissue.

"The males beat the hell out of each other," says Martin. "They are brutal. They can snap each other's jaws, tails, flippers, lacerate blowholes. The large males are literally covered with scar tissue." Only a small percentage of males turn bright pink, Martin says, and those are the ones females are most attracted to—at least during the mating season, when the water retreats back into the river channel and both sexes are thrown together.